The studies on Pseudomonas aeruginosa were designed to investigate the influence of extrinsic factors on the pathogenicity of this organism to provide insight into mechanisms by which infections they cause can be eliminated. P. aeruginosa was recovered from 26% (199) of 762 cancer patients. Recovery rates were highest in acute leukemias (52%) (P less than .001). Types 4 and 7 were frequently isolated from cancer patients but rarely from noncancer patients. Type 3 was more likely to be recovered from lymphoma patients and Type 6 from solid tumor patients. Colonization rate was highest in acute leukemias. Acute leukemic and CNS patients were colonized most often by Type 7, Hodgkins' non-CNS solid tumor and noncancer patients by Type 6, non-Hodgkins' lymphoma patients by Type 4 and myeloma patients by Type 2. P. aeruginosa were acquired by 65% of the patients (P less than .001). Type 7 was the most frequently acquired strain (P less than .004) and Type 2 was most commonly present at admission (P .01). Colonization and infection rates were higher with acquired strains. Over 65% of the colonized patients became infected. Acquired Types 1 and 7 caused more infections than did other types. Immunoglobulin levels to P. aeruginosa were highest in solid tumor patients and lowest in lymphoma patients. Ig level to the causative organism increased following urinary tract infections and pneumonias. Among solid tumor and lymphoma patients, the average titer was highest in non-colonized patients. It was highest in the non-colonized, non- infected leukemic patients. P. maltophilia and P. cepacia, which were frequently recovered from patients and their environs and were resistant to most antibiotics were susceptible to trimethoprim-sulfamethoxazole.